We have been studying the biochemistry and genetics of lambda site-specific recombination. We have purified some of the proteins involved in excisive site-specific recombination of bacteriophage lambda. We are studying the role of the phage-coded functions, Xis and Int, and host-coded functions in this recombination reaction. We have found, in vivo, that Xis and a host function, Him A, can partially substitute for each other in the recombination reaction. We are extending these studies to the in vitro recombination reaction.